Directional filters are used to filter light entering the filter from outside of a predetermined angle of incidence. These filters find particular use in cathode ray tube (CRT) heads down display apparatus in aircraft cockpits.
Heads down display apparatus of the type described is used to display, for example, a wide variety of aircraft navigational information in the cockpit of an aircraft for viewing by the pilot of the craft. Often the detailed nature of the displayed information makes it difficult to read under varying ambient light conditions. Consider, for example, that when ambient light is low, as in night flying, it is a relatively simple task to reduce the brightness of the display. On the other hand, there are frequently ambient light conditions which require a display brightness that would be impractical either as a result of the capabilities of the display or in consideration of the safety or comfort of the pilot-viewer. For example, if sunlight is creating a high glare condition, the display will not only have to overcome the glare, but must be bright enough for the displayed information to be discernible over background lighting conditions. The situation is magnified when it is considered that during the aircraft's maneuvering, ambient light can be expected to rapidly change.
The fixed position of the pilot-viewer enables the use of filter techniques which impart directional characteristics to ambient light. Accordingly, directional filters of various types have been disposed in front of the CRT face plate in order to block ambient light which tends to cause glare, while passing that light which is traveling in the direction of the pilot-viewer.
In one type of prior art directional filter, a sheet of material is etched to form a multiplicity of holes. The surface of the sheet at the holes has a high light absorbency characteristic which eliminates reflection along the holes and at the surface of the sheet. Frequently the sheets are stacked in order to enhance the light attenuation effect of the filter. Filters of this type are expensive to make and may have undesirably excessive light attenuation characteristics.
Another type of directional filter features a filter plate constructed from a plurality of thin sheets which are stacked so that each sheet is parallel to a direction of admitted light. The filter is formed by cutting a slice across the stack. This results in the filter being generally orthogonal to the direction of the individual sheets from which it is made, with the sliced direction varying from the orthogonal for central viewing angles which vary from the normal to the filter surface. Filters of this type are also expensive to produce.
Still another type of filter, such as described in commonly assigned copending U.S. application Ser. No. 476,297 filed by the present applicant on Mar. 17, 1983, is formed from a transparent plate having a plurality of etched grooves thereon. The grooves have a high light absorbency characteristic, particularly for light radiating toward the grooves from within the plate. Therefore, light entering the plate at an angle which causes the light to impinge upon the grooves is highly attenuated, whereas light entering the plate and not impinging upon the grooves is attenuated only by the optical characteristics of the plate. Filters of this type are somewhat more complicated and expensive to produce than is desired.
Accordingly, it is an object of the present invention to provide a directional filter for filtering ambient light which avoids the aforenoted disadvantages of prior art filters. Additionally, the filter of the present invention has relatively high light transmission capabilities and avoids secondary light reflections as will be understood upon reading the following description thereof.